Charge effects on the coalescence of water drops in free fall

Abstract

It has long been recognized that charge can affect the results of collisions in streams of water drops^1,2, between drops and a flat water surface³, between two drop streams^4–6, and for drops fired at suspended drops^7,8. Here we present the first observations of the effect of charge on the coalescence of colliding water drops that were initially falling freely at terminal velocity. Other experiments using uncharged drops show large discrepancies between drop streams or suspended drop results and free fall, terminal velocity results^9–11. Therefore, it is probable that previous work on charge effects should not be directly applied to clouds. The present findings support this contention, because the charge required to induce complete coalescence differs from previous results for similar drop sizes and size ratios by an order of magnitude. The following new features of charged drop interactions were observed: a unique impact angle dividing the coalescence from the non-coalescence region that is independent of charge, a two order of magnitude range of relative charge between drop bounce and complete charge-induced coalescence, a wide range of charge where a satellite drop occurs with temporary coalescence at higher charge levels the beginning of charge-induced permanent coalescence at the most grazing collisions.